Permission confusions with data import/export and running tempest

[orblivion]

I built Tempest, imported data from Sandstorm, and ran it. The README instructions as literally written give me permission errors. If I run import as sandstorm:sandstorm I can get import/export to work:

sudo -u sandstorm -g sandstorm ./_build/sandstorm-import-tool --snapshot-dir /tmp/sandstormexport export
sudo -u sandstorm -g sandstorm ./_build/sandstorm-import-tool --snapshot-dir /tmp/sandstormexport import

(Note that I'm putting it under /tmp/sandstormexport instead of ../sandstormexport because sandstorm doesn't have access to my user's home directory.)

But then if I try to run tempest I get this problem:

$ sudo -u sandstorm -g sandstorm ./_build/tempest
panic: mkdir /opt/sandstorm/var/tmp: permission denied

goroutine 1 [running]:
zenhack.net/go/util.Chkfatal(...)
	/home/user/go/pkg/mod/zenhack.net/go/util@v0.0.0-20230607025951-8b02fee814ae/chkfatal.go:6
sandstorm.org/go/tempest/internal/server/main.initStorage()
	/home/user/tempest/internal/server/main/init-storage.go:12 +0x8e
sandstorm.org/go/tempest/internal/server/main.Main()
	/home/user/tempest/internal/server/main/main.go:19 +0x34
main.main()
	/home/user/tempest/cmd/tempest/main.go:8 +0xf

If I run tempest as root it starts on port 80 and I get a 404.